This invention relates to theatre lighting in general and more particularly to an improved split cross fader control for use in controlling such lighting.
The advantages of voltage control of various types of devices is well recognized in the art. Through remote control by low voltage and low current signals, miniaturized controllers and sophisticated pre-processing of inputs is possible. Furthermore, the storage of any number of pre-program commands is possible. The usefulness of such control has been recognized in the theatre lighting art. Voltage controlled dimmers have been used with increasing regularity over the past 40 years. In particular, since the development of the thyristor in the late part of the 1950's, large scale usage of voltage controlled theatre lighting has become a reality. Through such voltage control, miniature control consoles or desks can be built to control up to several hundred dimmers. Typically in such consoles, dimmers are controlled by miniature potentiometers. It is evident that in order to accurately perform changes of more than a few dimmers would require the coordination of a large number of actions and becomes physically impossible. As a result, various forms of mastering, subgrouping and presetting have been devised for use in theatre lighting control.
Various types of systems are disclosed in a booklet entitled "Professional Lighting Control", published by Skirpan Electronics, 1968. This booklet discloses typical equipment available for theatre lighting control. In addition, the same company has developed a computerized system sold under the trademark "Autocue".
In a typical installation each lighting circuit to be controlled has associated therewith a dimmer control which is responsive to an analog input voltage to control the light to a level proportional thereto. Such dimmers are available from various manufacturers. In particular, they are available from the above-mentioned Skirpan Electronics. The desired lighting over the period of a program is predetermined and divided into what are referred to as cues or scenes. The conventional practice is to provide a plurality of potentiometers, one being provided for each circuit used in a cue or scene with the potentiometers being preset to provide the required analog voltage output to the respective dimmers. In some cases, the potentiometers are coupled to the dimmers through a patch panel to permit greater system flexibility. Also, in some systems the outputs of the individual dimmers are coupled to the lamps which they control through a further power patch panel to add further flexibility. These potentiometers in general terms comprise a memory system which records the desired levels for each cue or scene. In its simplest form, the memory in the prior art consisted of multiple potentiometers for each dimmer. One potentiometer per dimmer per scene is required. Thus, in a 12 dimmer, two preset console, 24 potentiometers would be required, i.e. 12 potentiometers for each scene. In an installation with 300 dimmers and having ten present scenes, the console would require 3000 potentiometers. In addition to this type of installation, other storage systems have been used including various forms of electrical and electromechanical memories. Such have been used with varying degrees of success. Most commonly in use today are potentiometers although in very large systems various types of electronic memories have been used such as in the above-mentioned computerized system where the cues are stored in the memory of a digital computer and converted to analog output voltages by means of digital to analog converters.
Presetting alone, however, will not provide as esthetically pleasing control of the theatre lighting. It is further necessary to be able to shift from one cue to the next in an orderly, smooth fade. Such is required because stage lighting can rarely be satisfactorily accomplished if all that is available are snap cues where the levels change instantaneously from one level to another. Basically, two approaches to this problem have been taken. One approach is through the use of what is referred to as a scene master. As noted above, cues in storage are usually referred to as scenes. The other approach is through the use of what is referred to as a cross fader. In the scene master approach, a master control is provided for each group of preset potentiometers or other storage devices. In some cases, subscene masters for controlling subgroups of dimmers which may be faded in and out of a scene separately are used. In this type of arrangement, the preset potentiometers or other memory devices obtain their inputs from the master control and provide an output, normally a DC voltage, having a magnitude proportional to the setting of the storage element multiplied by the setting of the scene master. In the art, the scene master setting is normally defined in terms of a decimal less than or equal to one or as a percentage. The value of the storage element is usually expressed as a number less than or equal to 100 or less than or equal to 10. For example, with a master set at 0.5 and a storage element or potentiometer set at 8 an output of 4 would result.
Thus, for each scene to be provided a scene master is installed having its output coupled to a set of preset potentiometers or the like. As noted above, the output of these preset devices is used as the dimmer control input. The outputs of the preset potentiometers for each scene are combined channel by channel and coupled to the dimmer in such a manner that the scene with the highest output will determine the output level of that channel or dimmer. Through this arrangement, an additional new scene can be constructed by bringing up more than one scene master at one time. An example of the manner in which this work is given in table I below. The rows labeled scene 1 and scene 2 give the values of the memory elements in the two scenes, each five dimmers. The row labeled pile-on scene is the new resulting scene which occurs when both scene 1 and scene 2 are brought up to a full at once. The last line in the table referring to a cross fader will be described below.
TABLE I ______________________________________ dimmers 1 2 3 4 5 Scene I 0 5 10 7 8 Scene II 9 4 0 7 10 Pile On Scene 9 5 10 7 10 Crossfader at 1/2 4.5 4.5 5 7 9 ______________________________________
Despite the advantages available in creating a pile-on scene, there are certain disadvantages in this approach. One of the primary disadvantages is that the fades of many dimmers will be non-linear during pile-on. This results because the output only reflects the highest level coming from the presets. For example, if scene 1 were at full and scene 2 were faded from zero to full, dimmer 1 would fade smoothly from zero to nine. However, dimmer 5 would remain at 8 until the output of scene 2 on channel 5 was greater than 8. Thus, this would occur only when the master reached a value of greater than 0.8 thereby causing dimmer 5 to fade up only in the last 20% of the master travel. A further disadvantage occurs where it is desired to fade one scene smoothly into the other. In such a case, one of the scenes, for example scene 2 have its master set at zero. The operator then lowers one master while raising the other. Ideally, what is desired is for the dimmers to slowly shift from the setting of one scene to that of the other. In the example of table 1 this will happen with the dimmers 1 and 3 but not in the other channels. Channel 4, which has the same value for both presets, would be expected to stay at that same value throughout the fade. Instead, if the masters are faded in such a way that both of them reach 0.5 at the same time the output of channel 4 will drop to 3.5 and then fade back up to 7. This problem is referred to as "fader dip". Thus, the scene master arrangement cannot provide smooth fading from one scene to the other in all instances.
The use of the cross fader is an attempt to overcome this problem. The cross fader is a single control which acts as a two sided master. By moving a control handle from one end of its travel to the other, one scene is faded in while the other is simultaneously faded out. For a two scene console the scenes are permanently assigned. In multiple scene units switching or patching is provided to enable each of the present scenes to be assigned to either end of the fader at will. In conventional terminology, the ends of the cross fader are referred to as the X and Y sides. A dipless cross fader must satisfy the following equation: EQU C.sub.1X . P.sub.X + C.sub.1Y . P.sub.Y = O.sub.1 and P.sub.Y + P.sub.X = 1
where:
C.sub.1x is the Channel One X scene value PA1 C.sub.1y is the Channel One Y scene value PA1 P.sub.x is the value of the preset X-side of fader PA1 P.sub.y is the value of the preset Y-side of fader PA1 O.sub.1 is the output to the dimmer.
The last line of Table I above shows what the cross fader outputs would be for the two listed scenes. Thus, at one half the cross fader will in channel 1 be at 4.5, halfway between zero and nine. Similarly, in channel 3 it will be at 5 halfway between zero and 10. Likewise channels 2 and 5 will be halfway between the two cues. Channel 4, which remains at 7 for both scenes, will be at 7 with the cross fader at half. Cross faders work quite well but have limited flexibility. This lack of flexibility causes problems in simple consoles with limted storage capacity. Specifically they do not have the capability of the scene master type of control where pile-on scenes can be created.
Thus, it is clear that the combination of the advantages of the scene master operation and cross fader operation would provide many useful benefits. Such has been attempted combining two scene masters next to each other and calling it a cross fader. However, such an arrangement does not cure the problem of dip referred to above. On the other hand, if a standard cross fader is built with two separate handles, it will operate properly for cross fades as long as it follows the equation given above. However, since both sides of the cross fader can be separately operated and can both be full at once, the second equation which must be satisfied, i.e. P.sub.X + P.sub.Y = 1 will no longer be true if both are at full. In such a case P.sub.Y + P.sub.X = 2. In such a case, if the scene 1 preset pot is at 5 and the scene 2 pot is at 6, then the pile-on of the two would be 11, much higher than either scene and in fact higher than the defined maximum value of 10. One attempt at solving this problem would be defining a split cross fader such that it followed the logic of a normal cross fader except that the output would be limited for each channel so that it would never exceed the higher of the two presets in use. Such a design would satisfy the end point condition for pile-on and would not dip during cross fades. However, pile-on fades of channels having non-zero values in both the X and Y presets would be non-linear. Channels that have higher settings on the preset already in use than on the preset to be piled-on will not change during a pile-on fade. Channels having a higher setting on the preset to be piled on will fade in during the earlier part of the fade. Such would be the case with dimmer 5 in Table I above. If scene 1 were at full, the output would be at 8. This would increase to 10 as the scene 2 fader was raised from zero. The fade from 8 to 10 would occur when the fader was raised from zero to 0.2. From 0.2 to full, no further change in the output would occur. Such as result is visually worse than the non-linearity of a scene master since the channel with the highest setting, hence the brightest lights, changes first. Similarly, removal of a piled on scene would produce just the opposite of fact, i.e., many dimmers will not fade down until the end of the fade out. Thus, it becomes clear that none of the presently available devices is capable of producing the combined type of operation desired and at the same time, providing smooth fading from scene to scene under all conditions. Thus, the need for such a device becomes evident.